1. Technical Field
The present invention relates to a method for producing a fiber laser.
2. Discussion
A fiber laser is an optical device comprising a doped optical fiber (active fiber) and a pump source adapted to provide a pump radiation to the doped optical fiber in order to excite the dopant. Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), Thulium (TM) and Praseodymium (Pr). The particular rare earth element or elements used is determined in accordance with the wavelength of the laser emission and the wavelength of the pump light.
The excited dopant tends to generate, as a consequence of its de-excitation, a stimulated emission radiation. The fiber laser also includes reflecting elements suitable to confine the stimulated emission radiation inside the optical fiber and to allow, when predetermined amplification conditions are reached, the output of part of this radiation. The reflecting elements may be, for example, Bragg gratings written on opposite ends of the doped optical fiber. A Bragg grating includes an alternation of zones with high refraction index and zones of low refraction index, mutually spaced at a distance that establishes the reflection wavelength (Bragg wavelength).
Bragg gratings are typically written in the core of standard transmission fibers, i.e. fibers not doped for stimulated emission purposes, in order to define reflecting elements for the transmission signals. Prior to the writing process on a standard fiber, a photosensitizer is usually added to the core of the fiber in the region predisposed to host the grating. The writing process then includes the exposure of the photosensitized region to a UV radiation, which generates an interferential pattern in the said core region inducing a refraction index variation. The interferential pattern may be obtained by means of different techniques, of which the most used are the xe2x80x9cphase maskxe2x80x9d technique and the technique consisting in focusing on the said core region two interfering UV beams.
Real time information on the grating characteristics during writing may be obtained by feeding to one end of the optical fiber the radiation of a wide spectrum source (e.g. a white light lamp or a LED) and by detecting, by means of a spectrum analyzer, the reflection spectrum at the same end of the fiber or the transmission spectrum at the opposite end of the fiber. This setup offers information on the peak wavelength, the intensity and the shape of the grating, and these information can be used to control process parameters like the UV intensity, the writing duration and the grating length.
A further technique to get information related to the writing process comprises feeding to one end of the fiber a wavelength tuned laser radiation and detecting the reflection optical power (at the same fiber end) or the transmission optical power (at the opposite fiber end) by means of a power meter. This further technique is slower but allows a higher resolution with respect to the previous one. Then, the first technique (wide band radiation feeding) is preferably used for real time monitoring of the writing process, while the second (tuned laser radiation feeding) is preferably used for grating characterization at the end of the writing process.
An easy technique to realize a resonant cavity on an active fiber comprises joining the active fiber with two stretches of not doped optical fiber each including a Bragg grating having a Bragg wavelength at the predetermined laser wavelength. However, the unavoidable insertion losses at the fiber joining region induces a laser power reduction and undesired reflections inside the resonant cavity which degrade the laser performances.
A different solution consists in writing the Bragg gratings directly in the core of the active fiber. In this case, the high absorption of the active fiber at the wavelengths of the radiation fed to the fiber (i.e. the wavelengths used for the real time monitoring or for grating characterization) makes the above mentioned monitoring techniques impracticable.
Typically, before writing a grating on an active fiber an evaluation of the required exposure time is made by considering data previously collected on identical but not doped fibers. However, no information is available in this way on the effective grating reflectivity obtained at the end of the writing process and, consequently, on the effective laser efficiency.
The document of Mikael Svalgaard, xe2x80x9cUltraviolet light induced refractive index structures in germanosilicaxe2x80x9d, Ph. D. thesis, March 1997, Mikroelektronik Centret, Published by Mikroelektronik Centret, Technical University of Denmark, Building 345 east, DK-2800 Lyngby, Denmark, depicts in Chapter 4 a work addressed to investigate the frequency stability of Er-doped fiber lasers that incorporate Bragg fiber gratings as the end mirrors. Svalgaard indicates that the dynamics of forming Bragg gratings involves spectral shifts of the same order of magnitude as the grating bandwidth (typically a fraction of a nanometer) and that such small changes during UV writing critically affect the performance of the resulting fiber laser. A method is proposed for real time monitoring of the laser performance based on simultaneous UV grating fabrication and pumping of the Er doped fiber.
In the description of the experimental setup (paragraph 4.2), a 10 cm Er-doped fiber is considered, whose ends are spliced to standard telecommunication fiber. The (first) grating formation dynamics is monitored in transmission using a broadband 1550 nm LED source. The first grating is exposed until the transmittance at the Bragg wavelength is 0.028xc2x10.001. During the writing of the second grating, Er-doped fiber is pumped by a 980 nm multimode diode laser through a 1530/980 nm wavelength-division multiplexing fiber coupler (WDM), and the laser output (near 1530 nm) is monitored on a spectrum analyzer. When the exposure time of the second grating approached that of the first, a maximum lasering power is reached. To prevent feedback optical isolators are used after both the diode and fiber lasers, and all fiber ends are angled. As reported in paragraph 4.3, to obtain robust single-frequency operation, the cavity must be very short. In the specific case, the cavity is 12.5xc2x11 mm long. Furthermore, according to Svalgaard, it is critical that the second gratings Bragg wavelength matches that of the first for lasering to occur.
The Applicant has observed that the fiber laser considered in the above document is a single-longitudinal-mode wavelength stabilized doped fiber laser, which includes an active fiber having a relatively low absorption at the Bragg wavelength, mainly due to the fact that the fiber is very short. This feature allows using a standard technique (feeding a wide band radiation to the fiber and detecting the related fiber output spectrum) for monitoring the characteristics of the first grating during the writing process.
The Applicant has noticed that, if a fiber laser has to be realized which includes an active fiber having a high absorption at the Bragg wavelength, the above method is no more suitable.
For the aim of the present invention, with xe2x80x9chigh absorptionxe2x80x9d it is intended an absorption of at least 15 dB in a range of about xc2x110 nm centered at the Bragg wavelength. The absorption of the fiber depends mainly on its geometry, on its length and on the dopant concentration.
The Applicant has in particular noticed that the first grating writing monitoring by means of a LED source or another wide band source would not be possible in case of a high absorption fiber, due to the excessive signal loss inside the fiber which would avoid correct spectrum detection.
Fiber lasers including a high absorption active fiber may be used, for example, as pump sources for optical amplifiers in optical transmission systems. For this kind of application, it is nor required to have a single-mode stabilized laser radiation and the active fiber is preferably designed so as to maximize the pump absorption. Then, the active fiber is preferably a relatively long and heavily doped fiber. It is further known that, in order to achieve a very high pump absorption, a fiber laser may advantageously include a double-cladding active fiber, i.e. an active fiber having a core for laser emission, an inner cladding larger than the core to receive the pump radiation and an outer cladding. The pump radiation is progressively transferred from the inner cladding to the core for dopant excitation. A fiber laser including a double-cladding active fiber is known, for example, from U.S. Pat. No. 5,530,709 in the name of SDL, Inc.
The Applicant has noticed that additional difficulties in the grating writing monitoring would arise if the considered active fiber is a double-cladding fiber. In fact, in this case the light of a wide band source will propagate mainly inside the inner cladding (having a geometrical section area much greater than the core) and the detected spectrum will then provide no indication on the grating written into the core. If the active fiber is a double-cladding fiber, the above condition on the absorption of the fiber is more pressing, and absorption values much lower than 15 dB (in a range of about xc2x110 nm centered at the Bragg wavelength) are sufficient to avoid the correct use of the known techniques.
The Applicant has found that a method for realizing a fiber laser including a high absorption active fiber comprises defining a reflecting surface associated to the active fiber and, during the first grating writing, pumping the active fiber in order to cause an amplified spontaneous emission between the first grating and the reflecting surface, and a consequent laser emission. This laser emission is detected and processed so as to allow a control of the laser performances during the writing process. The method then includes writing the second grating in a similar manner, where the resonant cavity is now defined between the first and the second grating.
The Applicant has found that, during the grating writing process, by repeatedly scanning the pump radiation fed to the active fiber between a minimum and a maximum value and by adeguately processing the output power from the active fiber, it is possible to derive real time values of the lasers efficiency and threshold power, which can be advantageously used to control the grating reflectivity in order to reach optimized laser performances.
According to a first aspect, the present invention relates to a method for producing a fiber laser, including writing a first grating having a first reflection wavelength band in an active fiber and includes the following steps:
defining a reflecting surface associated to the active fiber, before the step of writing said first grating, said reflecting surface having a second reflection wavelength band wider than the first reflection wavelength band;
optically pumping the active fiber, during the step of writing the first grating, in order to excite an amplified stimulated emission between the first grating and the reflecting surface and to consequently induce a laser emission from the active fiber;
measuring, during the step of writing the first grating, the optical power of the laser emission;
controlling the step of writing the first grating according to the measured optical power.
Preferably, the method includes writing in the active fiber, subsequently to the first grating, a second grating suitable to define, together with the first grating, a resonant cavity for said fiber laser.
The method preferably includes, during the step of writing the first grating, scanning the power of the pump radiation in a predetermined power range.
Preferably, the step of scanning is repeated with a predetermined scanning period and the step of measuring the optical power includes obtaining a predetermined number of optical power values during the predetermined scanning period.
The step of obtaining a predetermined number of optical power values preferably includes calculating, to obtain each of said optical power values, the average value of the optical power measured in a predetermined measuring period.
The method preferably includes the step of processing said optical power values in order to obtain a current value of the laser efficiency.
The step of processing preferably comprises finding a fitting line for a predetermined number of points on a laser gain characteristic corresponding to said optical power values, and evaluating the slope of said line.
The step of controlling the step of writing preferably includes checking if said current value of the laser efficiency has reached a limit value and, if said limit value has been reached, stopping the step of writing the first grating.
The step of checking preferably includes comparing said current value of the laser efficiency, related to a last scanning period, with a preceding value of the laser efficiency, related to a preceding scanning period.
The method preferably includes evaluating, according to said limit value, the reflectivity of said first grating.
The step of defining a reflecting surface preferably includes cutting and cleaning one end of the active fiber in order to define the reflecting surface at the interface glass/air.
The active fiber preferably has an absorption of at least 15 dB in a range of about xc2x110 nm centered at a wavelength corresponding to a maximum reflection wavelength of said first grating.
The active fiber preferably includes a double-cladding active fiber.
The method preferably includes the following steps:
optically pumping the active fiber, during the step of writing the second grating, in order to excite an amplified stimulated emission between said first and second gratings and consequently induce a laser emission from the active fiber;
measuring, during the step of writing the second grating, the optical power of the laser emission;
controlling the step of writing the second grating according to the measured optical power.
The method preferably includes, during the step of writing the second grating, scanning the power of the pump radiation in a predetermined power range.
The step of scanning is preferably repeated with a predetermined scanning period and the step of measuring the optical power preferably includes obtaining a predetermined number of optical power values during the predetermined scanning period.
The step of obtaining a predetermined number of optical power values preferably includes calculating, to obtain each of said optical power values, the average value of the optical power measured in a predetermined measuring period.
The method preferably includes the step of processing said optical power values in order to obtain a current value of the laser efficiency.
The step of processing preferably comprises obtaining from said optical power values a current value of the laser threshold power.
The step of controlling the step of writing preferably includes checking if said current value of the laser efficiency has reached a maximum value and, if said maximum value has been reached, stopping the step of writing the second grating.
The step of controlling the step of writing preferably includes stopping the step of writing the second grating when a predetermined relation between said current values of the laser efficiency and the threshold power has been reached.
The method preferably includes evaluating, according to said maximum value, the reflectivity of said second grating.
The method preferably includes, before the step of writing the second grating, defining a zone of negligible reflectivity in place of said reflecting surface.
Preferably, the second grating has a third reflection wavelength band and the ratio between the third and the first reflection wavelength bands is between 1,5 and 3.
According to a further aspect, the present invention relates to a fiber laser, including an active fiber, a first grating written in a first portion of the active fiber and having a first reflection wavelength band, and a second grating written in a second portion of the active fiber and having a second reflection wavelength band, the first and the secong grating defining a resonant cavity for the fiber laser, wherein in that the ratio between the widths of said first and said second reflection wavelength bands is between 1.5 and 3.
The active fiber preferably includes a double-cladding active fiber.
The active fiber preferably has an absorption of at least 15 dB in a range of about xc2x110 nm centered at a wavelength corresponding to the center of said reflection wavelength band.